This invention relates to electroluminescent display configurations, and more particularly relates to electroluminescence of semiconductor particles employed in an electroluminescent display configuration.
Electroluminescence is a well-characterized semiconductor phenomenon in which optical radiation, e.g., ultraviolet, visible, or infrared radiation, is generated by a radiative electron-hole recombination mechanism in a semiconductor material through which an electric current is passed under application of an electric field. The radiative electron-hole recombination mechanism is characterized, in a common manifestation of the electroluminescence phenomenon, by an interband transition between the conduction band minima and the valence band maxima of the semiconductor material, whereby the wavelength of luminescent radiation closely corresponds to the bandgap energy between the conduction and valence bands. It has been recognized that this wavelength-specific radiation phenomenon can be exploited to produce an electroluminescent optical display in which a semiconductor material is configured for generating a visible-light, optical display pattern. In one simple example, a powder of a semiconductor, e.g., ZnS, is embedded in a dielectric matrix such as plastic or glass to form an electroluminescent display; application of an alternating electric field to the embedded particles results in visible luminescence of the particles.
The electroluminescence properties of semiconductor particles, such as the ZnS powder just mentioned, have been of increasing interest due to the optical characteristics exhibited by semiconductor particles. Specifically, it has been found that the optical characteristics of semiconductor particles can be quite disparate from that of the corresponding bulk semiconductor material, with semiconductor particles exhibiting enhanced luminescence capabilities.
In this regard, silicon particles have been of particular interest. Bulk silicon exhibits only a very low level of electroluminescence due to its bandgap configuration. Bulk silicon is characterized by a bandgap energy of about 1.1 eV and a bandgap topology in which the conduction band minima and the valence band maxima are at differing wave vectors, i.e., the bandgap is indirect. As is well known, a radiative electron-hole recombination transition across this indirect bandgap does not conserve crystal momentum and thus the transition cannot occur unless a phonon or other scattering agent is provided for the required momentum conservation. The probability for this transition is thus extremely small unless extrinsic recombination centers are incorporated in the bulk material to enhance the radiative process; and as a result, the corresponding quantum efficiency of bulk silicon luminescent processes is quite low. Beyond the indirect nature of the bulk silicon bandgap, the relatively small energy of the bandgap results in a bulk silicon luminescent radiation wavelength that is in the infrared range, rather than more preferably visible range, of the electromagnetic spectrum.
It has been well-established that in great contrast with bulk silicon material, silicon particles exhibit electroluminescence at a range of wavelengths and do so with an enhanced quantum efficiency over that of the bulk material. These striking differences in electroluminescent performance are generally ascribed to two cooperating phenomena that are both characteristic of quantum effects enabled by the size regime of the silicon particles. With regard to a first of these quantum phenomena, it has been shown that silicon particles, as well as semiconductor particles in general, undergo a broadening of energy bands as the size of the particles is decreased. The bandgap of the particles correspondingly increases with the band broadening, with the specific bandgap of a particle depending on that particle's size. Based on this phenomenon, semiconductor particles, including silicon particles, can be engineered to provide a particle size regime corresponding to a band gap that enables a desired luminescence wavelength, including visible wavelengths.
With regard to the second of the quantum phenomena, it has been shown that the indirect nature of the bulk silicon bandgap is altered in correspondence with silicon particle size. Specifically, at nanometer particle sizes, the molecular orbital bonding arrangement characteristic of the particle size dictates a direct, rather than indirect, silicon bandgap. Nanometer-sized silicon particles have indeed been empirically found to have a bandgap that is shifted from that of an indirect topology to that of a direct topology. As a result, the electroluminesence quantum efficiency of silicon nanoparticles is understood to theoretically far exceed that of bulk silicon.
There have been proposed a wide range of configurations for exploiting the enhanced electroluminescence phenomena of silicon and other semiconductor particles, to produce a viable visible-wavelength electroluminescent display. For example, the bulk silicon topology known as porous silicon, produced by, e.g., anodic etching of bulk silicon, has been shown to have jagged columnar pores in the bulk material that can luminesce at a range of visible wavelengths, as described by Canham in Appl. Phys. Lett., V. 57, pp. 1046, 1990. It has been suggested that the columnar asperities of the pores luminesce in the manner of isolated nanometeric-sized particles. But porous silicon is found to be a suboptimal electroluminescent configuration in that the bulk silicon surrounding the columnar pores tends to absorb radiation produced at the pores, whereby the overall efficiency of the configuration is found to be relatively low. The silicon radiation absorption properties are also found to be wavelength specific, and to favor shorter wavelengths; as a result, even low efficiency porous silicon electroluminescence is found to be confined to the low energy, long-wavelength end of the visible spectrum.
A further proposed class of electroluminescent configurations is based on incorporation of isolated silicon or other semiconductor particles in an insulating layer such as silicon dioxide, as in the manner of the ZnS powder configuration mentioned above. The silicon dioxide layer maintains physical and electrical separation between the semiconductor particles and enforces an electron tunneling transport mechanism between the particles, as described by DiMaria et al., in "Electroluminescence studies in silicon dioxide films containing tiny silicon islands," J. Appl. Phys., V. 56, N. 2, pp. 401-416, July 1984. In this configuration, application of a voltage across a silicon dioxide layer containing silicon particles results in the probabilistic tunneling of injected electrons through the characteristically wide bandgap of the insulating layer from semiconductor particle to particle, with electron-hole recombination and corresponding luminescence occurring at the particles.
The electron tunneling phenomenon, known specifically as field-enhanced Fowler-Nordheim tunneling, that is enforced in this dielectric configuration sets an upper bound on the achievable luminescent quantum efficiency of the configuration. The process of electron tunneling is inherently inefficient, and is a highly probabilistic function of local bandgap conditions. As a result, the tunneling probability associated with the wide bandgap insulating layer, rather than the luminescence characteristic of the silicon particles, is found to be the dominant limiting factor in the overall quantum efficiency of this configuration. Thus, like the porous silicon configuration described above, the insulating layer configuration here limits the ability to exploit the silicon particle electroluminescence capabilities. Indeed, it has been found in general that electroluminescent configurations proposed heretofore have not provided an enabling mechanism for exploiting the high-efficiency luminescence associated with semiconductor particles in general and silicon particles in particular.